1. Field of the Disclosure
The present invention relates generally to a pulse width modulator, and more specifically, a pulse width modulator with a two-way integrator.
2. Background
A pulse width modulator (PWM) is a circuit that may be used in applications such as, but not limited to, motor control, switching power converters, or data transmission. A pulse width modulator may output a PWM signal that is a logic signal that switches between two logic states, such as a logic high state and a logic low state. In one design of a PWM, a capacitor may be used to integrate an input current representative of an input signal to determine a duty ratio during each sequential period of the PWM signal. The PWM signal is designed to vary the duty ratio according to one or more inputs. More specifically, the duty ratio may be defined as the ratio of time the PWM signal is in a certain logic state over a given time period. Typically, duty ratio is the amount of time the PWM signal is in the logic high state over a given period TS. A period TS, may be defined as the time duration of one complete cycle of the PWM signal. More specifically, a complete cycle of the PWM signal may be defined by the duration of time between when the PWM signal is switched to the first state and when the PWM signal is again switched to the first state.
A practical consideration in designing a PWM is determining the maximum duty ratio of the PWM signal. This can be important for many possible reasons. In the PWM that includes a capacitor to integrate, it may be necessary to control the maximum duty ratio such that enough time is available to allow the capacitor to reset (discharge), so it can be ready to integrate at the start of the next period TS. For example, if a maximum duty ratio is set to 99% while maintaining the PWM signal frequency above 66 KHz, the capacitor will only have 1% of the period, which is 150 ns, to reset the capacitor before start of the next period TS.
To further complicate the issue, the design of the capacitor used may also have a non-linearly changing capacitance at low voltages. In order to maintain proper functionality of the pulse width modulator, an offset to the voltage range in which the capacitor is allowed to integrate is implemented such that integration of the input current occurs where the capacitor value operates in a linear range. For example, due to the nature of the materials used for capacitors in an integrated circuit, the capacitor may integrate inconsistently when a voltage across the capacitor is under 1V, and thus the capacitor may only integrate starting from an offset voltage of 1V. This prevents the capacitor from using a ground node (0 V) and being reset to zero V. Therefore, the capacitor must not only be able to reset within a short time frame because of a high duty cycle (i.e 99%), but also may need to reset to a pre-determined set voltage reference (i.e 1V) to avoid integrating in a non-linear region of the capacitor.
In one example, an LED (light emitting diode) light source is powered by a source of dc power. Because power is generally delivered through a wall outlet as high-voltage ac power, a device, such as a power converter, is required to transform the high-voltage ac power to usable dc power for the LED light source. In operation, a power converter controller, included in the power converter, may output a PWM signal to drive a power switch of the power converter to control the amount of power delivered to the LED light source. In one example, feedback information representative of the output voltage and/or output current at the LED light source may be input to the PWM to adjust the duty ratio of the PWM signal. In this manner, a desired output voltage, output current, and/or output power at the output of the power converter may be regulated.